1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof. In particular, the present invention relates to a wiring connection structure of a semiconductor integrated circuit device in which a silicon layer, such as a doped polysilicon layer or a doped amorphous silicon layer, and a metal silicide layer or a metal layer are connected to each other in a connection hole.
2. Background Art
In semiconductor devices, circuit elements such as transistors are usually formed on a semiconductor substrate. Various kinds of interconnections are formed on a semiconductor substrate to electrically connect between the internal circuit elements as well as to electrically connect the circuit elements to external circuits. Among various kinds of interconnections, the polycide lead which consists of a doped polysilicon layer, to which an impurity such as phosphorus is added to reduce resistance, and a metal silicide layer such as a tungsten silicide layer is used for various kinds of interconnections as typified by a gate electrode because of its relatively low wiring resistance and superior heat resistance. With recent increases in integration densities and operation speeds of semiconductor devices, it is increasingly desired to connect between polycide leads each other with as low a contact resistance as possible.
FIG. 9 is a sectional view showing a conventional semiconductor device, particularly an example of a connection between its polycide lead layer.
As shown in FIG. 9, an underlying insulating film 2 is formed on a silicon substrate 1, and a gate insulating film 3 is formed on the underlying insulating film 2. A first polycide lead layer 4 is formed on the gate insulating film 3. The first polycide lead layer 4 consists of a first doped polysilicon layer 5 and a first tungsten silicide layer 6.
After an interlayer insulating film 7 is formed on the gate insulating film 3 so as to cover the first polycide layer 4, an opening 8 is formed through the interlayer insulating film 7 so as to reach the top of the first tungsten silicide layer 6 of the first polycide lead layer 4. A second polycide lead layer 12 consisting of a second doped polysilicon layer 9 and a second tungsten silicide layer 10 is connected to the first polycide lead layer 4 in the opening 8. The first and second polycide lead layers 4 and 12 are connected to each other substantially through contact between the first tungsten silicide layer 6 and the second doped polysilicon layer 9.
To reduce the contact resistance between the first and second polycide lead layers 4 and 12, the second doped polysilicon layer 9 is doped with phosphorus atoms 11. A semiconductor device including the above structure will be subjected to subsequent various heat treatment steps to be completed as a final product.
In the conventional semiconductor device including the above structure, there occurs, in the opening 8, a phenomenon that the phosphorus atoms 11 that are added in the second doped polysilicon layer 9 diffuse into the first tungsten silicide layer 6 in each heat treatment step of the manufacturing process.
As a result, a portion of the second doped polysilicon layer 9 near the interface with the first polycide lead layer 4, from which portion the phosphorus atoms 11 have escaped, comes to have high resistivity. Further, the contact between the second doped polysilicon layer 9 and the first tungsten silicide layer 6 makes a transition from ohmic contact to Schottky contact, because the difference in work function increases as the concentration of the phosphorus atoms 11 in the second doped polysilicon layer 9 decreases.
For the above reasons, the semiconductor device including the structure of FIG. 9 has a problem that the contact resistance between the first and second polycide lead layers 4 and 12 is very high.
FIG. 10 is a sectional view showing an example of a conventional semiconductor device in which the above-mentioned problem is solved.
As shown in FIG. 10, an opening 8 penetrates through the first tungsten silicide layer 6 of the first polycide lead 4, and the first and second polycide lead layers 4 and 12 are connected to each other through contact between the first and second doped polysilicon layers 5 and 9.
In this structure, the phosphorus atoms 11 do not move between the first and second doped polysilicon layers 5 and 9 if the difference in phosphorus concentration between the first and second doped polysilicon layers 5 and 9 is made small. Therefore, the contact resistance remains low.
However, to realize the semiconductor device including the structure of FIG. 10, in forming the opening 8 both of the interlayer insulating film 7 and the first tungsten silicide layer 6 need to be etched.
However, there is a problem that according to the current etching technology there is great technological difficulty in forming an opening through both of the interlayer insulating film 7 and the first tungsten silicide layer 6 by a single etching step.
On the other hand, if openings are formed in the interlayer insulating film 7 and the first tungsten silicide layer 6 by separate etching steps, there arises a problem that the manufacturing cost increases.